The invention relates to a current source, which may, for example, be used as part of the control circuitry for display devices, and particularly display devices having current-driven pixels, for example active matrix electroluminescent display devices. Such display devices comprise an array of electroluminescent display pixels arranged in rows and columns, and the invention also relates to display devices using such current sources.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional III-V semiconductor compounds. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of a semiconducting conjugated polymer sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer.
The polymer material can be fabricated using a CVD process, or simply by a spin coating technique using a solution of a soluble conjugated polymer. Organic electroluminescent materials exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore be used in passive type displays. Alternatively, these materials may be used for active matrix display devices, with each pixel comprising a display element and a switching device for controlling the current through the display element. Examples of an active matrix electroluminescent display are described in EP-A-0653741 and U.S. Pat. No. 5,670,792, the contents of which are incorporated herein by way of reference material.
A problem with display devices of this type arises from the fact that they have current driven display elements. Conventional supply circuitry for supplying a controllable current to the display elements can suffer the drawback that the current varies as a function of the electrical characteristics of the switching transistors used in the supply circuitry. For example, a current controlling transistor may be provided as part of the pixel configuration, with the gate voltage supplied to the transistor determining the current through the display element. Different transistor characteristics give rise to different relationships between the gate voltage and the source-drain current. Such an arrangement is described in EP-A-0653741.
The current controlling circuitry may either comprise part of the pixel configuration, as described above, so that a pixel voltage is supplied to the pixels, or else the current controlling circuitry may comprise separate circuitry provided at the periphery of the display area, so that a pixel current is supplied to the pixels. In either case, if the current controlling circuitry is integrated onto the same substrate as the display pixels, it typically comprises thin film switching elements such as thin film transistors. The uniformity across the substrate of the electrical characteristics of the switching elements may be poor, which gives rise to unpredictable variations in the pixel currents and therefore the pixel outputs.
According to the invention, there is provided a display device comprising:
an array of pixels arranged in rows and columns, each pixel comprising a current-driven display element;
driver circuitry for generating signal currents corresponding to desired outputs from the display elements, characterised in that the driver circuitry comprises means for generating a predetermined voltage function, the voltage function being supplied to the input of a binary weighted capacitor network, each capacitor of the network being connected in series with a respective switching transistor, an input word to the driver circuitry controlling the switching of the switching transistors, such that the output current of the driver circuitry is a function of the voltage function and of the combined capacitance of the capacitors of the network for which the associated switching transistor is closed, the output current being supplied to a selected pixel of the array.
The current output of this arrangement depends upon the voltage across the capacitors of the network, and the combined capacitance. In particular, the current through each capacitor is a function of the rate of change of voltage across the capacitor. The currents through each capacitor having a closed switch are summed to provide the current output. The voltage across each capacitor depends on the voltage function, and can be substantially independent of the characteristics of the switching transistors. In particular, the voltage across the switching transistors (i.e. the source-drain voltage) will become constant once the transistors have saturated, so that the rate of change of voltage across the capacitors is then independent of the voltage drop across the switching transistors. Therefore, non-uniformity between different switching transistors will be of no consequence.
The voltage function may comprise a ramp function, so that a constant current through the capacitors results (after an initial settling period). However, other functions may be used, which may be selected to give an output current which itself has a desired function over time.
The display device preferably further comprises row addressing circuitry for addressing rows of the array in turn, and the driver circuitry comprises a binary weighted capacitor network for each column of the array, and provides a respective signal current to each column during a row addressing period.
Each pixel may comprise first and second switching means, and may be operable in a first mode in which the input current is supplied by the first switching means to the second switching means, a control level being stored for the second switching means corresponding to the input current, and in a second mode in which the stored control level is applied to the second switching means so as to drive a current corresponding to the input current through the display element. The display element is preferably an electroluminescent display element. By using the current supplied by the driver circuitry to configure the pixel, this pixel is made to mirror the input current.
The second switching means may then comprise a TFT, and the gate-source voltage of the TFT at an operating point in which the source-drain current is the input current is stored on a capacitor as the control level.
The output current may be sampled to control a current source, rather than being supplied directly to the pixel. The current source then supplies the current to the selected pixel of the array. This avoids the need to apply a constantly increasing voltage as the voltage function for the full duration of the row address period.
The invention also provides a current source for providing a current output based on a binary input control word, comprising:
a binary weighted capacitor network; and
means for generating a predetermined voltage function, the voltage function being supplied to the input of the binary weighted capacitor network,
wherein each capacitor of the network is connected in series with a respective switching transistor, the binary input word controlling the switching of the switching transistors, such that the output current of the driver circuitry is a function of the voltage function and of the combined capacitance of the capacitors of the network for which the associated switching transistor is closed, the output current being supplied to an output of the current source.
This current source may be used in the display device of the invention, but it may also be used in other applications requiring a constant current input.